Chemical Fertilizer Reduction Combined with Biochar Application Ameliorates the Biological Property and Fertilizer Utilization of Pod Pepper

Abstract

Biochar is frequently utilized as a helpful amendment to sustain agricultural productivity. However, it remains uncertain whether biochar can effectively replace chemical fertilizers, especially in karst regions. To investigate the effects of co-applying biochar and chemical fertilizer on the biological characteristics and fertilizer uptake of pod peppers, as well as to determine the optimal ratio of biochar to chemical fertilizers, a two-year field experiment was conducted in southwest China. The results showed that, compared to the locally typical chemical fertilizer treatment (CF), the combined application of biochar and chemical fertilizer significantly increased the yield of both fresh and dry pod pepper. Chemical fertilizer reduction and biochar application also ameliorated fruit quality, increased nutrient accumulation, and improved fertilizer utilization efficiency. What is more, although the employment of biochar made production costs higher, the reduction in chemical fertilizers and the increase in yield improved economic efficiency, especially in the CF₇₀B treatment (70%CF + biochar). In conclusion, moderate amounts of biochar instead of chemical fertilizers may be a valid nutrient management strategy for pod pepper in the karst mountain areas, which is beneficial for maintaining yield stability, improving quality, and increasing net income.

1. Introduction

As an essential guarantee of food security and the adequate supply of agricultural products, chemical fertilizers play an irreplaceable role in agricultural production [1]. About 200 million tons of chemical fertilizers are used per year in agriculture worldwide, making up more than 40% of the world’s grain output [2]. China is the world’s biggest user of chemical fertilizers, whose consumption of nitrogen, phosphorous, and potassium fertilizers is over 20% of the world’s total [3]. The high utilization of chemical fertilizers in agricultural production is vital for a continuous increase in grain production. According to statistics, the total vegetable production in China exceeded 700 million tons in 2020, which is more than 50% of global vegetable production [4,5]. However, because of the objective factors of poor essential soil fertility and a sizeable replanting index, most of the high yields of vegetables have resulted from the massive application of chemical fertilizers, and the coverage of scientific fertilization techniques is low [6,7]. The overuse of chemical fertilizers leads to serious secondary soil salinization, negatively impacting arable land quality, solidifying soil over time, destroying soil structure, and significantly impeding crop growth. This disruption of crop growth hinders the yield and quality of vegetables, ultimately damaging the entire farmland ecosystem [8,9]. Moreover, a great number of chemical fertilizer nutrients accumulated in the soil or lost to the environment cause the serious waste of resources and environmental pollution [10,11,12]. Reducing the use of chemical fertilizers and improving their efficiency are necessary requirements for achieving environmentally sustainable agricultural growth. Therefore, it is essential to optimize fertilizer management in China to enhance both vegetable production and environmental sustainability.

The “Zero Growth of Fertilizer Use” action initiative was created and implemented by the Chinese government in 2015 to minimize the excessive use of chemical fertilizers [13]. Substituting organic chemical fertilizers has emerged as a vital technique for reducing fertilizer usage and enhancing soil quality [14,15,16]. High temperatures (about 700 °C) and low oxygen levels are required to generate biochar [17,18]. Moreover, due to its rich nutritional composition, biochar can heal deteriorated soil and improve soil physicochemical qualities (such as strengthening water retention, raising pH, increasing porosity, and decreasing soil bulk density) [19,20,21]. Therefore, biochar applied to soil as an organic fertilizer or amendment is considered a new generation of green products that can replace chemical fertilizers. Sun et al. [22] found that biochar (10–20 t·ha⁻¹) could take the place of chemical N fertilizer (20–40%) and stabilize the yield of vegetables via increasing nutrient use efficiency and improving soil quality under field conditions, which matched with the yield of the complete rate N application treatment. Gu et al. [23] pointed out that the use of biochar together with a 20–40% chemical fertilizer reduction was beneficial to ameliorate soil properties without reducing rice yield, which can be an effective strategy to maintain sustainable soil productivity in rice production systems. Therefore, biochar has been utilized as both a technical tool to reduce fertilizer usage and boost efficiency, and as an amendment to enhance the soil environment.

Research has recently focused on exploring the potential role of biochar as a substitute for chemical fertilizers, using a combination of biochar and reduced chemical fertilization. Studies have shown that the use of biochar, (20–40 t·ha⁻¹) combined with 20% nitrogen (N) fertilizer reduction, could promote crop root development and yield formation, and could improve microbial diversity and the content of total organic carbon (TOC) in macro-aggregates in upland purple soils [24,25,26]. According to An et al. [27], replacing a moderate amount of chemical fertilizer with biochar may enhance the soil’s physical structure, boost rice productivity, and reduce fertilizer input. Furthermore, studies have demonstrated that the use of biochar, in combination with reduced fertilizer application, can facilitate sustainable nitrogen and phosphorus utilization in soil through several conversion processes [28]. Although several researchers have studied the effect of biochar with chemical fertilizer reduction on crop production potential [27,29,30], the proper amount of biochar to replace chemical fertilizer remains unclear because of differences in crop types, test sites, and soil conditions.

Pod pepper holds a vital place among economic crops, as it is not only the largest vegetable crop in terms of planting area and production value in southwest China, but also boasts a high nutritional value [31,32]. Numerous field experiments have demonstrated that supplementing chemical fertilizers with biochar can enhance crop yield and soil nutrient levels. Nevertheless, there is a research gap concerning the effects of biochar supplementation and reduced chemical fertilization on pod pepper’s biological properties and on fertilizer utilization in karst mountainous regions. To evaluate the possibility of replacing chemical fertilizer with biochar in southwest China, a two-year field experiment was conducted to assess the impact on the pod pepper’s productivity.

2. Materials and Methods

2.1. Site Description

The 2-year field experiment was carried out in Shangji Town (27°23′45′′ N, 106°59′59′′ E), Bozhou District, Zunyi City, Guizhou Province, China, from 2021 to 2022. This area has an average of 891 mm rainfall per year and an average annual temperature of 14.3 °C. The experimental region’s soil type is yellow soil, which is prevalent in the karst mountains of southwest China.

Table 1. The basic physicochemical properties of soil and biochar.

	pH	OM (g·kg−1)	TN (g·kg−1)	TP (g·kg−1)	AP (mg·kg−1)	TK (g·kg−1)	AK (mg·kg−1)
Soil	6.26	21.33	1.19	-	29.85	-	202.97
Biochar	8.23	425.05	39.84	9.29	-	19.88	-

Note: OM stands for organic matter. TN stands for total nitrogen. TP stands for total phosphorus. AP stands for available phosphorus. TK stands for total potassium. AK stands for available potassium.

displays the physicochemical characteristics of the surface soil (0–20 cm) within the experimental site.

2.2. Experimental Material

The pod pepper variety used in the experiment was ‘Zunla 9′, bred by Zunyi Academy of Agricultural Sciences, and the main local planting variety. The chemical fertilizers employed in the experiment included CF1 (N-P₂O₅-K₂O 15-14-16, N in CF1 is ammonium nitrogen, Guizhou Tianbao Fengyuan Ecological Agricultural Technology Co., Ltd., Xiuwen, China) and CF2 (N-P₂O₅-K₂O 18-6-18, N in CF2 is ammonium nitrogen, Guizhou Tianbao Fengyuan Ecological Agricultural Technology Co., Ltd., Xiuwen, China). Distillers’ grains from Guizhou Moutai Brewery (Group) circular economy Industrial Investment Development Co, Ltd., Zunyi, China, was utilized as the raw material for biochar production through the oxygen-limited cracking method in the biomass carbonization furnace (SSDP-5000-A, Jiangsu Huaian Huadian Environmental Protection Machinery Manufacturing Co., Ltd., Huaian, China). A predetermined amount of distillers’ grains was added to the furnace and purged with N2 for approximately 5–10 min to remove excess air. The material was then cooled, pyrolyzed at 550 °C for 2 h, passed through a 100-mesh sieve, and kept in a cool place until ready for use. The physical and chemical properties of the resulting biochar are presented in Table 1.

2.3. Experimental Design

In 2021–2022, a field trial that involved two pepper-planting seasons was carried out. There were six treatments: (1) CK: no fertilizer application; (2) CF: local customary chemical fertilizer application without biochar, 900 kg·ha⁻¹ CF1 + 900 kg·ha⁻¹ CF2; (3) CF₉₀B: 10% chemical fertilizer reduction combined with biochar, 810 kg·ha⁻¹ CF1 + 810 kg·ha⁻¹ CF2 + 3000 kg·ha⁻¹ biochar; (4) CF₈₀B: 20% chemical fertilizer reduction combined with biochar, 720 kg·ha⁻¹ CF1 + 720 kg·ha⁻¹ CF2 + 3000 kg·ha⁻¹ biochar; (5) CF₇₀B: 30% chemical fertilizer reduction combined with biochar, 630 kg·ha⁻¹ CF1 + 630 kg·ha⁻¹ CF2 + 3000 kg·ha⁻¹ biochar; and (6) CF₆₀B: 40% chemical fertilizer reduction combined with biochar, 540 kg·ha⁻¹ CF1 + 540 kg·ha⁻¹ CF2 + 3000 kg·ha⁻¹ biochar.

In the experiment, CF1 and biochar were applied to the soil as based fertilizer before transplanting pod pepper. Then, a rotary tiller was used to mix fertilizers and the soil evenly. Fifteen days after ridging and covering with plastic film, the pod pepper seedlings were transplanted. The planting density was 45,000 plants·ha⁻¹. Each treatment was performed randomly within a block and repeated three times, with the plot size measuring 35.00 m² (3.5 m × 10.0 m). CF2 was applied to the soil as topdressing fertilizer at the early flowering stage of pod pepper. In addition, no irrigation was applied, and the same fungicides, pesticides, and herbicides were applied as needed.

2.4. Soil Sampling and Analysis

Soil samples (0–20 cm in depth) were taken at 15 randomly chosen sites using a soil auger prior to fertilizers being applied. The soil samples were added together to form a composite sample, following which they were air-dried and ground. The resulting mixture was sieved through 1.00 mm and 0.15 mm sieves to assess pH, soil organic matter (SOM), total nitrogen (TN), available phosphorus (AP), and available potassium (AK). Soil pH was determined using a 1:2.5 extraction mixture and a pH meter (FE20K, Mettler Toledo, Zurich, Switzerland). SOM was determined using the high-temperature external heating potassium dichromate oxidation volumetric method. TN was analyzed by H₂SO₄-H₂O₂ digestion Kjeldahl method. AP was measured by extraction with molybdenum antimony anti-colorimetry, and 0.5 mol·L⁻¹ NaHCO₃. AK extraction was performed with ammonium acetate and measured using a flame photometer (FP640, Shanghai Aopu Analytical Instrument Co., Ltd., Shanghai, China) [33].

2.5. Plant Sampling and Analysis

Six plants were sampled in each plot when the pod pepper was ripening. The fresh pod pepper plants were divided into three parts: stems, leaves, and fruits. The plant samples were first baked at 105 °C for 0.5 h, then baked at 60 °C to a consistent weight. Then, the weighed dried stems, leaves, and fruits were added to measure the aboveground dry biomass of pod pepper. N, P, and K concentrations were then determined by grinding the dried samples, putting them through a 0.25 mm screen, and then digesting them with a concentrated H₂SO₄ and H₂O₂ solution [31]. In addition, samples of fresh pod pepper from each plot were collected at the mature stage to determine the free amino acid content, reducing sugar, VC, and nitrate [31].

2.6. Yield of Pod Pepper

The yield of fresh pod pepper in each plot was weighed according to the maturity of pod pepper in each plot. Then, the final yield of fresh pepper was calculated according to the weight of multiple harvests. In addition, the moisture content of fresh pod pepper collected each time was calculated after being brought back to the laboratory for drying. Then, the yield of dry pod pepper was calculated.

2.7. Calculations and Statistical Analysis

The parameters were calculated using the methods described by Zhang et al. [32].

2.7.1. Nutrient Accumulation

The calculation method for nutrient accumulation is as follows.

$$\mathrm{N}\mathrm{A}=\mathrm{N}\mathrm{C}\times \mathrm{D}\mathrm{B}$$

where NA stands for nutrient accumulation (kg·ha⁻¹), NC stands for nutrient concentration (%), and DB stands for dry biomass (kg·ha⁻¹).

2.7.2. Fertilizer Utilization

The calculation method for fertilizer utilization is as follows.

$$\mathrm{A}\mathrm{E}=\frac{{\mathrm{Y}}_{\mathrm{F}}–{\mathrm{Y}}_{\mathrm{C}\mathrm{K}}}{{\mathrm{N}\mathrm{I}}_{\mathrm{F}}}$$

$$\mathrm{R}\mathrm{E}=\frac{{\mathrm{N}\mathrm{A}}_{\mathrm{F}}–{\mathrm{N}\mathrm{A}}_{\mathrm{C}\mathrm{K}}}{{\mathrm{N}\mathrm{I}}_{\mathrm{F}}}$$

AE represents the agronomic efficiency (kg·kg⁻¹), Y_F represents the dry yield of fertilization treatment (kg·ha⁻¹), Y_CK represents the dry yield of the control treatment (kg·ha⁻¹), RE represents the recovery efficiency (%), NA_F represents the nutrient accumulation of fertilization treatment (kg·ha⁻¹), NA_CK represents the nutrient accumulation of control treatment (kg ha⁻¹), and NI_F represents the nutrient input of fertilization treatment (kg·ha⁻¹).

2.7.3. Economic Benefits

The calculation method for economic benefits is as follows.

$$\mathrm{O}\mathrm{V}=\mathrm{Y}\times \mathrm{U}\mathrm{P}$$

$$\mathrm{N}\mathrm{E}\mathrm{I}=\mathrm{O}\mathrm{V}-\mathrm{F}\mathrm{V}$$

where OV stands for the output value (CNY·ha⁻¹), Y represents the yield of dry pod pepper (kg·ha⁻¹), UP represents the unit price of dry pod pepper (CNY·kg⁻¹), NEI represents the net income (CNY·ha⁻¹), and FV represents the fertilizer value (CNY·ha⁻¹). The unit of dry pod pepper price was 20.00 CNY·kg⁻¹ when calculating the economic benefits. The CF1, CF2, and biochar fertilizers were 3350, 3500, and 2000 CNY·t⁻¹, respectively.

2.8. Statistical Analysis

The results are presented as mean ± standard error, and statistical analysis was conducted using one-way ANOVA and Duncan’s technique for multiple comparisons, with a significance level of p < 0.05, using SPSS 18.0 (SPSS Inc., Chicago, IL, USA). Origin 8.0 (Origin Lab Corporation, Northampton, MA, USA) was used to prepare the figures.

3. Results

3.1. Effects of Fertilization Management on Pod Pepper Yield

Chemical fertilizer application improved the output of fresh pod pepper by 77.48–135.99% (2021) and 73.75–209.81% (2022) in comparison to the CK treatment (Figure 1). The output of fresh pod pepper improved by 6.61–32.97% (2021) and 63.20–78.31% (2022) when chemical fertilizer use was reduced, and biochar treatments were used. In both years, the CF₇₀B treatment produced the highest yield of fresh pod pepper, resulting in 15,027 kg·ha⁻¹ yield in 2021 and 18,393 kg·ha⁻¹ yield in 2022. The dry pepper yield showed a similar pattern to the fresh pepper yield. Chemical fertilizer decrease and biochar treatments led to a considerable increase in dry pod pepper output of 6.85–26.76% in 2021 and 34.61–50.08% in 2022 compared to the CF treatment. In both years, the CF₇₀B treatment produced the highest yield of dry pod pepper, resulting in 3293 kg·ha⁻¹ yield in 2021 and 3592 kg·ha⁻¹ yield in 2022.

3.2. Effects of Fertilization Management on Fruit Quality of Fresh Pod Pepper

The application of different fertilization methods has a significant impact on fresh pod pepper fruit quality (

Table 2. Effects of chemical fertilizer reduction combined with biochar on the quality of fresh pod pepper.

Year	Treatments	Free Amino Acid (g·kg−1)	Reducing Sugar (mg·kg−1)	VC (g·kg−1)	Nitrate (mg·kg−1)
2021	CK	3.86 ± 0.11 a	28.35 ± 3.33 b	0.82 ± 0.07 c	83.36 ± 1.47 b
	CF	3.87 ± 0.09 a	28.56 ± 1.99 b	0.89 ± 0.08 bc	88.76 ± 1.95 a
	CF90B	3.90 ± 0.10 a	30.12 ± 2.92 b	0.97 ± 0.08 ab	88.37 ± 2.39 a
	CF80B	3.95 ± 0.09 a	40.42 ± 4.72 a	1.10 ± 0.06 a	85.15 ± 2.61 ab
	CF70B	4.00 ± 0.11 a	40.55 ± 2.13 a	1.06 ± 0.08 a	84.13 ± 2.14 b
	CF60B	4.00 ± 0.10 a	39.04 ± 2.63 a	1.03 ± 0.07 a	75.44 ± 1.84 c
2022	CK	3.70 ± 0.08 b	30.39 ± 1.40 c	0.69 ± 0.08 e	82.43 ± 2.09 a
	CF	3.74 ± 0.10 b	31.06 ± 0.37 c	0.83 ± 0.08 d	86.81 ± 2.17 a
	CF90B	3.79 ± 0.12 ab	40.88 ± 3.64 b	1.05 ± 0.09 c	66.35 ± 1.62 b
	CF80B	3.80 ± 0.09 ab	47.89 ± 4.32 a	1.26 ± 0.08 a	64.49 ± 2.30 bc
	CF70B	3.96 ± 0.09 a	47.93 ± 3.46 a	1.21 ± 0.07 ab	58.82 ± 6.44 c
	CF60B	3.81 ± 0.14 ab	44.58 ± 1.88 ab	1.11 ± 0.08 bc	49.34 ± 5.60 d

Note: CK—no fertilizer; CF—100% chemical fertilizer; CF₉₀B—90% chemical fertilizer + biochar; CF₈₀B—80% chemical fertilizer + biochar; CF₇₀B—70% chemical fertilizer + biochar; CF₆₀B—60% chemical fertilizer + biochar. Using Duncan’s MRT approach, different lowercase letters in the same column show significant differences between treatments at the p < 0.05 level.

). Compared with the CF treatment, chemical fertilizer combined with biochar treatments increased the content of free amino acids by 0.78–3.36% in 2021 and 1.34–5.88% in 2022. However, the content of free amino acids did not show a significant difference in 2021 but showed an evident distinction in 2022. The chemical fertilizer decrease and biochar treatments raised the reduced sugar content by 5.46–41.98% in 2021 and by 31.62–54.31% in 2022 compared to the CF treatment. The reduced sugar content in the CF₈₀B and CF₇₀B treatments was the highest over the two years. Meanwhile, chemical fertilizer reduction and biochar treatments increased the VC content by 8.99–23.60% in 2021 and 26.51–51.81% in 2022. Furthermore, incorporating biochar, in conjunction with reduced chemical fertilizer application, significantly lowered the nitrate levels in fresh pod pepper fruits by 0.44% to 15.01% in 2021 and 23.57% to 43.16% in 2022. Notably, the CF60B treatment had the smallest amount of nitrate in both years.

3.3. Effects of Fertilization Management on Nutrient Accumulation of Pod Pepper

The nutrient accumulation of N, P, and K in pod pepper plants is shown in Figure 2. Compared with the CK treatment, using chemical fertilizer increased the N, P, and K accumulation by 82.52–153.35%, 21.72–51.14%, and 32.13–73.64%, respectively, in 2021, whereas it increased by 112.53–209.40%, 25.19–95.45%, and 54.61–105.38%, respectively, in 2022. Compared with the CF treatment, the N, P, and K accumulations of the chemical fertilizer reduction combined with biochar treatments increased by 12.39–38.81%, 7.87–24.18%, and 8.33–31.41%, respectively, in 2021, whereas they increased by 15.77–45.58%, 18.75–56.12%, and 11.54–32.84%, respectively, in 2022. The CF₇₀B treatment had the most excellent N, P, and K accumulation over the two years, with values of 135.18, 28.66, and 231.26 kg·ha⁻¹ in 2021 and 129.58, 34.19, and 220.68 kg·ha⁻¹ in 2022, respectively.

3.4. Effects of Fertilization Management on Fertilizer Utilization of Pod Pepper

The fertilizer’s effectiveness was greatly boosted by reducing the chemical fertilizer and using charcoal (

Table 3. Effects of reduced chemical fertilizer combined with biochar on the fertilizer utilization of pod pepper.

Year	Treatments	Agronomic Efficiency (kg·kg−1)			Recovery Efficiency (%)
		AEN	AEP	AEK	REN	REP	REK
2021	CK	—	—	—	—	—	—
	CF	3.23 ± 0.48 c	12.21 ± 1.81 c	3.78 ± 0.56 c	14.83 ± 1.00 d	5.24 ± 1.38 d	16.86 ± 2.61 d
	CF90B	4.84 ± 0.34 bc	18.29 ± 1.29 bc	5.66 ± 0.40 bc	24.63 ± 2.16 c	9.07 ± 2.85 c	32.55 ± 3.60 c
	CF80B	6.50 ± 0.41 ab	24.55 ± 1.56 ab	7.60 ± 0.48 ab	33.42 ± 2.24 b	13.77 ± 2.29 b	41.57 ± 3.16 b
	CF70B	7.96 ± 1.58 a	30.08 ± 5.96 a	9.31 ± 1.84 a	39.36 ± 3.56 a	17.63 ± 0.08 a	55.18 ± 1.52 a
	CF60B	6.38 ± 2.56 ab	24.12 ± 9.67 ab	7.47 ± 2.99 ab	31.48 ± 4.32 b	12.58 ± 0.98 b	37.71 ± 3.73 bc
2022	CK	—	—	—	—	—	—
	CF	3.27 ± 1.64 c	12.37 ± 6.21 c	3.83 ± 1.92 c	15.87 ± 2.25 d	5.61 ± 2.16 d	23.11 ± 0.73 d
	CF90B	6.74 ± 1.31 b	25.46 ± 4.94 b	7.88 ± 1.53 b	25.90 ± 4.78 c	12.03 ± 3.73 c	40.92 ± 3.39 c
	CF80B	8.75 ± 1.13 ab	33.05 ± 4.27 ab	10.23 ± 1.32 ab	34.49 ± 1.76 b	20.84 ± 0.94 b	50.65 ± 4.73 b
	CF70B	10.44 ± 1.21 a	39.47 ± 4.58 a	12.22 ± 1.42 a	42.18 ± 1.97 a	30.35 ± 2.54 a	63.71 ± 2.19 a
	CF60B	10.54 ± 0.95 a	39.82 ± 3.61 a	12.32 ± 1.12 a	34.32 ± 3.78 b	24.08 ± 0.59 b	51.10 ± 1.94 b

Note: AE_N, AE_P, and AE_K stand for agronomic efficiency of N, P, and K. RE_N, RE_P, and RE_K stand for recovery efficiency of N, P, and K. CK—no fertilizer; CF—100% chemical fertilizer; CF₉₀B—90% chemical fertilizer + biochar; CF₈₀B—80% chemical fertilizer + biochar; CF₇₀B—70% chemical fertilizer + biochar; CF₆₀B—60% chemical fertilizer + biochar. Based on Duncan’s MRT method, the appearance of different lowercase letters in the same column indicates significant distinction among different treatments at a level of p < 0.05.

). In comparison with the CF treatment, the AE_N, AE_P, and AE_K of decreased chemical fertilizer together with biochar treatments increased by 49.85–96.59%, 49.80–146.36%, and 49.74–146.30%, respectively, in 2021, whereas they increased by 106.12–222.32%, 105.82–221.91%, and 105.74–221.67%, respectively, in 2022. The CF₇₀B treatment showed the highest AE in 2021, whereas CF₇₀B and CF₈₀B were the highest in 2022. Compared with the CF treatment, the REN, REP, and REK of the biochar treatments, combined with reduced chemical fertilizers, increased by 66.08%–165.41%, 73.09%–236.45%, and 93.06%–227.28%, respectively, in 2021. Meanwhile, these measures increased by 63.20%–165.78%, 114.44%–441.00%, and 77.07%–175.68%, respectively, in 2022. The RE_N, RE_P, and RE_K in the CF₇₀B treatment were 39.36%, 17.63%, and 55.18%, respectively, in 2021, and 42.18%, 30.35%, and 63.71%, respectively, in 2022.

3.5. Effects of Fertilization Management on Economic Benefits of Pod Pepper

Table 4. Effects of chemical fertilizer reduction combined with biochar on economic benefits of pod pepper.

Year	Treatments	Output Value (CNY·ha−1)	Fertilizer Input (CNY·ha−1)	Net Income (CNY·ha−1)
2021	CK	32,761 ± 4033 d	—	32,761 ± 4033 d
	CF	51,952 ± 1825 c	6165	45,787 ± 1825 c
	CF90B	58,641 ± 3404 bc	11,549	47,092 ± 3404 bc
	CF80B	63,626 ± 2200 ab	10,932	52,694 ± 2200 ab
	CF70B	65,855 ± 4282 a	10,316	55,539 ± 4282 a
	CF60B	55,513 ± 5633 c	9699	45,814 ± 5633 c
2022	CK	31,261 ± 3834 d	—	31,261 ± 3834 d
	CF	52,656 ± 7007 c	6165	46,491 ± 7007 c
	CF90B	70,881 ± 4201 b	11,549	59,332 ± 4201 b
	CF80B	76,974 ± 2741 ab	10,932	66,042 ± 2741 ab
	CF70B	79,029 ± 3704 a	10,316	68,713 ± 3704 a
	CF60B	72,570 ± 3743 ab	9699	62,871 ± 3743 ab

Note: CK—no fertilizer; CF—100% chemical fertilizer; CF₉₀B—90% chemical fertilizer + biochar; CF₈₀B—80% chemical fertilizer + biochar; CF₇₀B—70% chemical fertilizer + biochar; CF₆₀B—60% chemical fertilizer + biochar. Duncan’s MRT method demonstrates that distinct lowercase letters within the same column indicate statistically significant differences between treatments at p < 0.05.

displays the effect of biochar and reduced chemical fertilizer use on the financial advantages of pod pepper. The output value of dry pod pepper in the biochar treatments combined with reduced chemical fertilizers increased by 3561–13,903 CNY·ha⁻¹ and 18,225–26,373 CNY·ha⁻¹ for 2021 and 2022, respectively, compared to the CF treatment. This reflects an increase of 6.85% to 26.76% in 2021 and 34.61% to 50.09% in 2022. The output value of the CF₇₀B treatment was the highest in the two years, at 65,855 CNY·ha⁻¹ in 2021 and 79,029 CNY·ha⁻¹ in 2022. The net income per hectare of dry pod pepper increased by 27–9752 CNY·ha⁻¹ and 12,840–22,222 CNY·ha⁻¹ in 2021 and 2022, respectively, in the biochar treatments combined with reduced chemical fertilizers compared to the CF treatment. The increased rates were 0.06% to 21.30% in 2021 and 27.62% to 47.80% in 2022. The net income of the CF₇₀B treatment among all treatments was the highest in the two years, at 55,539 CNY·ha⁻¹ in 2021 and 68,713 CNY·ha⁻¹ in 2022.

4. Discussion

The outcomes of this study suggest that the utilization of biochar, in combination with reduced chemical fertilizers, has the potential to enhance both fresh and dry pepper yields (Figure 1). This conclusion is consistent with findings from earlier research. Studies have shown that biochar can improve soil quality, which solves the problem of soil consolidation due to excessive fertilizer application and achieves increased crop yield [34,35,36]. On the one hand, biochar possesses a high specific surface area, and is also rich in mineral components that enable it to absorb soil nutrients directly, thereby reducing nutrient loss [37,38]. On the other hand, biochar can stimulate the activity of soil microorganisms and improve the soil microbiological environment, which is beneficial for promoting crop root growth and yield formation [39,40,41]. The findings of the present investigation suggested an inhibition of yield at the highest doses of mineral fertilizer (CF90B). This may be related to the toxicity of ammonia. On the one hand, the nitrogen in the fertilizer used in the experiment was ammonium N, and the excess ammonium N may produce ammonia toxicity to inhibit plant growth [42,43]; on the other hand, biochar may have inhibited nitrification, which may further enhance the toxicity of ammonia [44,45]. In addition, with the reduction in N application, the ammonia toxicity will diminish so that plant growth will not be inhibited. Remarkably, although low-dose chemical fertilizer application combined with biochar (CF60B) could improve pepper yield, it may also restrict crop growth and decrease yield compared to the CF₇₀B treatment. For example, An et al. [27] found that substituting chemical fertilizers with biochar could have limitations in proportion and that extended application duration could alleviate these adverse effects [46,47].

Improving the quality of agricultural products is at the core of high-quality agricultural development. Many studies have confirmed that chemical fertilizers with biochar can significantly improve the nutritional quality of crops while increasing crop yield [48,49]. In this study, chemical fertilizer reduction combined with biochar increased free amino acids, reducing sugars, and VC content in fresh pepper fruits in comparison with the CF treatment. It significantly reduced the nitrate content in fruits (Table 2). Due to the regular fertilization and continuous release of nutrients, biochar application facilitates the coordination and balance of crop nutrient metabolism, which contributes to improved fruit quality [50,51]. In addition, it was found that the improvement in crop quality might be because the utilization of biochar raised the photosynthetic rate of leaves and facilitated the transport of photosynthetic products to the fruit, which is also beneficial for fruit quality [52,53]. Moreover, biochar boosts the adsorption of NH₄⁺ and decreases the conversion of NH₄⁺ to NO₃⁻, which facilitates the reduction in nitrate content [54,55].

Fertilizer utilization efficiency is an essential indicator for evaluating fertilizer application measures. Studies have demonstrated that the judicious use of biochar enhances soil nutrient status and fosters a more diverse and abundant microorganism population, thus promoting the improvement in soil micro-ecological environments and nutrient utilization efficiency [56,57,58]. Table 3 shows that the application of biochar resulted in a remarkable increase in AE and RE compared to the use of chemical fertilizers alone. These findings imply that, in the specific setting of this investigation, decreasing the usage of chemical fertilizers and incorporating biochar can form a promising strategy to improve fertilizer utilization efficiency. Studies have shown that biochar utilization increases the soil C/N ratio and limits nitrogen conversion and denitrification by soil microorganisms, which facilitates the sequestration of more NH₄⁺ and NO₃⁻ in the soil [33,59,60]. In addition, biochar can be used as an alternative to traditional phosphorus fertilizers because of its inherently high content of effective phosphorus, and it can impact and alter the cycling and effectiveness of phosphorus in soils through the adsorption and desorption of phosphorus and the regulation of the soil microbial community structure [61,62,63]. Similarly, except for the direct contribution of potassium to biochar, biochar can also increase the effective soil potassium content and improve potassium fertilizer use efficiency by stimulating microbial activity [64,65].

Biochar holds excellent potential for use as an effective and cost-efficient soil amendment [34,66]. The biochar treatment combined with reduced chemical fertilizers resulted in an increase in net income of 0.06% to 21.30% in 2021 and 27.62% to 47.80% in 2022, as observed in this study (Table 4), which indicated that the utilization of biochar did not decrease the economic efficiency of pod pepper under the condition of chemical fertilizer reduction, but was beneficial to economic efficiency. Phares et al. [67] also found that, owing to its affordable production cost, NPK fertilizer reduction, combined with biochar, may increase the yield of maize grain and the net revenue. However, biochar is a powdered particle that may migrate with water or float on the soil surface when applied to the soil, potentially threatening the environment and human health [68,69]. Therefore, it is imperative to seek a material that can immobilize biochar powder and improve its adsorption performance in future research. This will solve the complex problem that powdered biochar can easily migrate and improve its adsorption capacity with higher economic and environmental benefits [70,71,72]. In addition, cheaper biochar materials or preparation technologies are critical directions for future research, which may be more conducive to reducing the input cost of biochar in agricultural production and thus achieving better economic returns.

5. Conclusions

This field research showed the possibility of improving pod pepper productivity, ameliorating fruit quality, and increasing fertilizer utilization and economic benefits by combining biochar and decreasing chemical fertilizers. Under the experimental conditions, 3.0 t·ha⁻¹ biochar application can replace 30% of chemical fertilizers to maintain pod pepper’s high productivity and financial benefits in the karst region. However, the rate of biochar replacement with chemical fertilizers should not be too high, which may be detrimental to productivity, owing to insufficient NPK nutrients. Moreover, the field effects of biochar replacement with chemical fertilizers may be strongly associated with the quantity and duration of biochar application. Therefore, the mechanisms of improving the quality and efficiency of pod pepper in karst mountainous areas by reducing chemical fertilizer and using biochar require further investigation.